John Mueller, a Google Webmaster Trends Analysts, confirmed with me on Twitter that Google did send out these notifications yesterday to sites that have implemented their hreflang markup incorrectly.

Hreflang markup is designed for websites that serve users from around the world with content translated or targeted to users in a certain region. Google uses the rel=”alternate” hreflang=”x” attributes to serve the correct language or regional URL in search results. If you mark up your pages incorrectly, it can confuse Google, and now Google is sending out a large set of those notifications.

Google sent this notification first back in July 2015, but, as I said, there are tons of webmasters who have said they received them for the first time yesterday.